I have a Toshiba split unit 18000 BTU A/C system using R410A, what pressure should the low side be please?

Question

I have had the unit for a while and the cooling seems to be less effective than it used to be (filters are all clean etc etc).

I know that if it has lost gas it needs to be reclaimed and re-filled because it is a blend - however as they guys have a call-out charge of £50 - plus £15 min labour - and I have a 410A compatible manifold (high pressure friendly plus 410A adaptor), I was rather hoping to check the pressure myself.

I can hook the manifold gauge up to the charging port with the unit off, in cool mode or in heat mode. What I don't know is what pressure I should be looking at to indicate that the unit is either low / borderline or fine.

I have found a pressure chart for 410A BUT I do not know if this is the correct pressure when the unit is off / running cold, running hot - or has nothing to do with the correct pressure of the low-side at all.

If the system NEEDS more gas, I will get the folks over - just loathed to spend £65 for someone to spend 3 minutes to poke a gauge on and say "nah, that's fine mate, that's £50 call-out and £15 for the first 15 mins Guv :)

Very many thanks

Mark

Answer 1

Generally speaking you should see 110 psi to125 psi on the low side to have a coil temp of 35 deg F to 45 deg F.

The unit should be running at least 10 min.

Evaporator coil temp drop would be an easier way to check since pressures themselves depend on what load and pressure drop it has.

15 deg. F - 20 deg F  of temperature drop from the intake to the outlet would be normal. On the lower end if the humidity is high and a higher end if the humidity is low.

If you only have 10 deg difference your low on refrigerant.

Answer 2

The gauge is a guide for the service Tech, more important is the temp of the suction line at a given pressure. The suction line should not be beer can cold say 40 degrees, this tells you that it is overcharge or a air flow problem. The suction line should not be too warm which could mean low charge. The Tech has to read the pressure and temp of the suction line at the same time, then look at the charts inside the equipment to see if it is normal, if he does not do this than he is not doing the procedure right. So if you have a service Tech who uses only his gauges find another Tech who will do the job the right way!

Answer 3

Under normal conditions a 118 low pressure and a 395 head pressure should be expected.

Answer 4

Mark the low side pressure will change as the air temp through your indoor coil changes. if your building is warm and moving warm air through the unit, the low side could be cooling but it is starting at a warm temp so the pressure and temp are directly preportinal.

Looking at your low side gage installed on the low side port (and running) at least ten minutes to settle down. And with your Return air temp at factory test conditions of 75 degrees F your indoor coil temp will be 40 degrees. look for the temp scale on your gage line see where the neddle lines up the 410 scale 40 degree mark and then look at what pressure the needle is pointing. Your outside air temp can change pressures .also. usually there is a chart and charging method on the unit or in the manual. You really need to know what your head pressure is, your wet bulb temp entering the unit/ the outdoor temo/ the leaving air rise/ th suction line temp and the liquid line temp and so forth to match it with factory specs. And remember, if it is short of gas, It should not be..You would have a leak and it would need to be found and repaired too.